There are many examples of such machines which essentially comprise a rigid supporting framework within which is resiliently mounted a box or basket containing one or more mesh screens onto which the mixture of solids and liquids is poured. To assist in the sieving action it is known to vibrate the box or basket so that the solid matter on the mesh screens is itself caused to vibrate. This causes clumps of fine material to break up, and if appropriate, pass through the mesh, together with the liquids, and for larger pieces of material which cannot pass through a mesh to migrate over the surface of the mesh to form part of the solids output of the machine.
By using a coarse mesh screen followed by a fine mesh screen so the process of screening is split into two steps in which liquid and fines (small solid particles) pass through the coarse mesh leaving the larger solid material thereon, and essentially the liquid alone is able to pass through the very fine mesh screen leaving the fines to be discharged from the second screen.
U.S. Pat. No. 4,167,478 describes a modular screening machine having at least one screen for separating particulate material according to size in which the screen box is driven so as to describe an elliptical movement at one end and a substantially rectilinear oscillatory movement at the other end.
U.S. Pat. No. 4,340,469 also describes a vibrating screening apparatus containing two screens which are vertically spaced above and below two independently rotatable shafts containing unbalanced weights so that when rotated the screen assembly is caused to move in a generally elliptical path. By choosing the position of the two shaft axes relative to the centre of gravity of the assembly, a rocking movement is introduced at the infeed end of the assembly.
U.S. Pat. No. 5,265,730 describes a further arrangement in which rotary eccentric vibrators are mounted outboard of the machine but act on the screen basket in such a way as to introduce an appropriate shaking movement.
In such machines the objective is twofold. On the one hand the material has to be vibrated so as to separate liquid from solids as far as possible, and to separate small solids from larger solids particularly where a degree of adhesion can exist as a result of the inherent characteristics of the materials concerned. On the other hand as soon as the liquid and solid separation has occurred, it is desirable to remove the solid material from the screens as quickly as possible to make way for further material which is usually desirably delivered for processing on a continuous basis.
Generally movement of solid material off the screen is achieved by incorporating into the motion a backwards and forwards component, as is known in the field of conveyors. This will cause particulate material to migrate along the length of a conveyor in response to an appropriate shaking movement. A drive for such a conveyor is described in U.S. Pat. No. 4,212,731 which also utilises two counter-rotating eccentric weights to generate the forces necessary to introduce conveyance of particulate material along the surface of the conveyor illustrated therein.
Whereas the rapid removal of solid material from the screen is obviously advantageous where a high throughput is required, if the rate of removal is too high the migrating solids and fines may carry too much of the liquid phase.
Whilst it has been found that improved efficiency, both in terms of screening and throughput, can be achieved using two screens, a first having a relatively coarse mesh and a second having a relatively fine mesh as aforesaid, the actual movement of each of the two screens in order to achieve optimum processing is not necessarily the same for each screen.
It has been proposed to drive the two screens separately so as to achieve optimal movement of each screen. Such an arrangement however introduces additional complexities and cost and whilst it does allow for optimisation of a screening process, is not always desirable.
The present invention seeks to provide a simple and expedient solution by providing an improved vibratory drive and filter basket constructions for a sieve particularly a shale shaker as aforesaid, to obtain a high consistency in the ratio of solids transported therethrough.
The invention provides an improved form of such a machine which is more suitable for real time control using closed loop feedback systems for optimising filtering and throughput.
The invention also provides an improved shale shaker which can be used as a basic unit or in conjunction with other similar units to provide for greater throughput, and to permit a reliable and controllable screening of mixtures of solids and liquids, with or without real time closed loop feedback control.